Modelling the morphodynamic impact and the effect on significant wave height due to sand extractions : a model study for the Holland coast area

This thesis presents a study into both the morphodynamic effects as well as the hydrodynamic effects in terms of the significant wave height of a sand extraction in the coastal area of the Holland coast. Due to the reduction of available sand and possible cost reductions, Rijkswaterstaat is interested in the effects of new sand extractions strategies on the safety of the coast, i.e. larger extractions in terms of depth as well as extractions closer to the shore in the Holland coast part of the Dutch coast than current regulation allows.
For the morphodynamic impact of a sand extraction also referred to as a sand pit, we built a process-based idealized sand pit model (referred to as the 2015 sand pit model), based on a previous sand pit model (referred to as the 2008 sand pit model). The 2015 sand pit model is used for flow, sediment transport and bed evolution in a tide-dominated environment. The 2015 sand pit model will enable a multi-directional basic flow, where the 2008 sand pit model used a unidirectional basic flow. The multi-directional flow is a more realistic representation of the tidal flow and allows for tidal ellipses with arbitrary orientation, amplitude, and eccentricity. Tidal current data is gathered from the MATROOS model of Rijkswaterstaat and a harmonic analysis is performed with the MatLab program t_tide to reproduce a realistic tidal signal.
For the impact of a sand pit on the significant wave height we use the Delft3D-WAVE model with storm conditions corresponding to recurrence times of 100, 1,000, and 10,000 year in the Holland coast. These storm conditions are water level, significant wave height, peak period, wind speed, wind direction, and wave direction. The Delft3D-WAVE model is used to simulate the evolution of wind-generated waves in coastal waters. The model computes wave propagation, non-linear wave-wave interactions and wave dissipation, for a given bottom topography and water level. We choose to omit the morphodynamic effects during storm conditions and only look at the change in significant wave height. Multiple bed profiles are gathered from bathymetry data of the Holland coast and are used to make a schematized two-dimensional bed profile. A sensitivity analysis for both models is carried out to analyse the effects of pit geometry and position on the model results.
The morphodynamic effects in the 2015 sand pit model are presented in terms of area of morphodynamic influence, pit deepening, radius of morphodynamic influence and migration. The morphodynamic effects after 50 years are found to be small. Furthermore, the 2015 sand pit model results show that sand pits trigger the morphodynamic instability associated with the formation of large-scale bed features known as tidal sandbanks. A gradual deformation of the pit itself can be observed, as well as the appearance of adjacent humps. The corresponding time scale is of the order of decades to centuries, where the time-scale decreases for pits at smaller water depths. Finally the migration rate is independent of the pit geometry and depends only on the tidal flow conditions, this agrees with previous findings.
The change in significant wave height in the Delft3D-WAVE model is found to be stronger for deeper pits and pits closer to the shore with the used model settings. A decrease in significant wave height is found landward of the pit due to diffraction. Areas with an increase in significant wave height are found locally next to pit. The increase in significant wave height in the BCL zone is negligible and in the rest of the coastal foundation the relative increase is in the order of a few percent.
It can be concluded with the used models and underlying assumptions that both large scale sand extractions at the 20 m NAP depth line and small scale sand extractions closer to the shore
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result in relatively small changes with respect to the situation without a sandpit. It has to be mentioned that the 2015 sand pit model predicts tidal sand banks whereas in the study area shoreface connected ridges are present with a different orientation. It would be recommended to add wind driven sediment transport, which enables for the growth of shoreface connected ridges.